Bogen CHB 100 into guitar amp

[TerryD]

Are these good to turn into guitar amps?  Some people seem to question the 100. 

Whats the easiest way to turn this into a guitar amp?  I'm very beginner but have done filter caps etc.  Mostly worked on old fenders with the "dummy" picture schematics.  Any help as always is appreciated.  Terry

[jjasilli]

Lot's of info already posted here.  From a cursory check, it looks like all the Bogen CHB models share the same preamp.  The PI and power amps are different to generate the different output powers, which I think are 10, 14, 20, 35, 50 & 100W in the CHB line.

100W is a lot.  20W is enough for me.

The CHB preamps use a James tonestack - a version of the Baxandall.  That's fine for a PA amp, but way too dark for me as an electric guitar amp, though some people (I think Tubenit, e.g.) like it.  I use a Bogen CHB-20 with the stock tonestack for acoustic guitar performance.

[Jack_Hester]

I use a Bogen CHB-20 with the stock tonestack for acoustic guitar performance.

Is your amp stock, or modified?  The reason I ask is that a good friend of mine plays mostly acoustic.  His primary acoustic has a piezoelectric pickup under the bridge, and feeds into one of his pedals that he designed to play though our Church sound system, the pedal being powered by the phantom voltage on the line.

I don't believe that he has a good acoustic amp, so I'm wondering if this is a good circuit to build from, as he doesn't always have a mixing board to connect his pedal.  A good acoustic amp would serve him well. 

Sorry to hyjack the thread. 

Jack

[jjasilli]

My Bogen CHB-20 is stock, except: I added a phono plug input in place of the Amperex screw on mic input; and a speaker-out phono plug.  The +Lug on the speaker-out jack is wired to the stock wire jumper.  To change speaker impedance I move the jumper wire from one screw to another on the stock terminal strip. Here's my thread: on this topic:  http://www.el34world.com/Forum/index.php?topic=15868.0

Later I added an outboard DIY attenuator in order to lower the vol. of the monitor speaker from the house speaker:  http://www.el34world.com/Forum/index.php?topic=16161.0

This system sounds great -- lot's of compliments on sound quality.

[jojokeo]

For guitar amp converting, the tone stack but mainly the preamp stage(s) biasing can make a big difference.

[Platefire]

All the CHB models are good for guitar amp conversions. I've converted a 10A, 20A and a 50. The only thing is a 100 would be a lot more power than I would ever want. 50 watts is about max for me. Also the transformers are a little undersized and Bogen used a Voltage Doubler system to get the voltage up to the needs of the circuit. So I think if you convert it to guitar, you need to leave the power supply set up the same way. Platefire

[kagliostro]

Here the CHB100 - CHB 100B Schematics

and a link to a tread talking about a conversion

http://www.diytube.com/phpBB2/viewtopic.php?f=8&t=4771

K

[clayman]

A great amp to learn and tweak,lots of room. I pulled two power tubes,put in a Marshall tonestack,and a long tailed phase inverter. In hind sight,it makes a better acoustic amp. Oh and a master volume is necessary.

[SoundmasterG]

I think most of the Bogen stuff was rated peak power when they were quoting their power numbers. The CHB100 is more like 50 to 60 watts rather than the 100 they claim. Even if you compare the specs for a 7868 in the RCA tube manual, they quote a max of 44 watts RMS for a pair, so that would be only 88 watts, and the Bogens use undersized OT's so they won't be able to make 100 watts of power. The one I gutted and modified was 54 watts RMS before I put power scaling in.

They're cool amps to play around with, but they do things different than the normal Fender such as the voltage doubler. Their layout is poor on the stock chassis and the transformers are too close to each other and will cause hum if you increase gain a lot like I did over the stock setup.

Greg

[jjasilli]

FYI re Bogen power output:  I have Manuals for the Bogen CHB20A, 35A, 50 & 100, printed in 1962 - 1972.  They state Useable Power: 20W, 35W, 50W & 100W, respectively, all @ 5% distortion.  This indicates that Bogen was stating RMS, not peak power.  But I haven't tested it. 

[PRR]

A healthy Bogen will make its badge-plate number in RMS Watts at large (but not gross) distortion. And all day long.

My 150s would do 140W very clean and 160W barely bent. I tested mine many-many hours. Because I got them after one (of six) *caught fire* in a full-time FULL-power operation (driving projector motors off-speed). I eventually found an un-soldered connection in the one that burned. (It hung on the mechanical crimp for several years before letting go.)

With up-rated filter caps mine would do 180W.

IMHO, any Bogen not making rated power isn't being tested right, or is sick.

[SoundmasterG]

PRR, how do you explain a CHB100 claiming 100 watts RMS and it uses a quad of 7868's which state in the RCA tube manual that 450V on the plate, 400V on the screens, and a 6600 ohm plate to plate impedance will make 44 watts for a pair, thus 88 watts for a quad into 3300 ohms plate to plate impedance? The CHB100 OT is 2500 ohms impedance and B+ in mine was about 480V. Mine made 54 watts RMS into a resistive load before I added power scaling, and there is plenty of drive in this amp to overdrive the power tubes if needed. Considering a Twin Reverb makes 80 watts or so with similar voltages and 6L6GC's, I think 100 watts is a bit optimistic by Bogen....and I don't think the CHB100 I modified is malfunctioning when it only produces 54 watts RMS.

Greg

[jazbo8]

The CHB100 OT is 2500 ohms impedance and B+ in mine was about 480V. Mine made 54 watts RMS into a resistive load before I added power scaling, and there is plenty of drive in this amp to overdrive the power tubes if needed. Considering a Twin Reverb makes 80 watts or so with similar voltages and 6L6GC's, I think 100 watts is a bit optimistic by Bogen....and I don't think the CHB100 I modified is malfunctioning when it only produces 54 watts RMS.

Greg

Which tap did you use for the dummy load during your test? Po = 54W for a quad 7868/7591A seems way too low...

[PRR]

> state in the RCA tube manual

Tube manual numbers are "suggestions". They may not be very-best, because they want to sell you a costlier tube, or they fear a bad design will leave a bad impression.

Also some of the tube-manual suggestions show quite low THD. 6L6GC shows 56 Watts max at low THD, a good safe clean condition. But in a restricted market (ham radio), G.E. published a paper showing 6L6GC making nearer 70 Watts at higher voltage and 5%-10% THD. Tube life may not be great in constant full-output operation, but hams speak in bursts and tolerate some distortion.

> that 450V on the plate, 400V on the screens, and a 6600 ohm plate to plate impedance will make 44 watts for a pair, thus 88 watts for a quad into 3300 ohms plate to plate impedance? The CHB100 OT is 2500 ohms impedance and B+ in mine was about 480V.

480V/450V, squared, is 1.138 times the power. Already at 100.12 Watts.

3300/2500 suggests 1.32 times the current (IF the tube will support it). Looks like 132 watts.

Take off 10% for OT loss, 118 Watts. Probably some B+ sag and maybe the tubes don't pass 1.32 times current, so slightly over 100W.

> Mine made 54 watts RMS into a resistive load

I'd re-test that. IF I really cared 50W or 100W. (Usually if 50W won't do the job, 100W isn't total bliss, so maybe not worth the effort.)

Bogen "had" to hit their specified numbers, because fixed-install contractors tended to own Test Gear (to impress customers, or just to play with) and maybe 10% of them actually load-tested the power amps they installed. In the fixed-install racket the equipment specs are considered almost as "sure" as a contract for 10,000 bricks. OK, you might overlook 9,999 bricks or 99W. But 5,400 bricks, or 54 Watts, would be cause for rejection.

[smackoj]

Bogen "had" to hit their specified numbers, because fixed-install contractors tended to own Test Gear (to impress customers, or just to play with) and maybe 10% of them actually load-tested the power amps they installed. In the fixed-install racket the equipment specs are considered almost as "sure" as a contract for 10,000 bricks. OK, you might overlook 9,999 bricks or 99W. But 5,400 bricks, or 54 Watts, would be cause for rejection.
[/quote]

PRR has me laughing today, "to impress or just to play with"  I think this one-liner could be in a book of famous quotes somewhere?

 thx for the laugh hombre         

[SoundmasterG]

> state in the RCA tube manual

Tube manual numbers are "suggestions". They may not be very-best, because they want to sell you a costlier tube, or they fear a bad design will leave a bad impression.

Also some of the tube-manual suggestions show quite low THD. 6L6GC shows 56 Watts max at low THD, a good safe clean condition. But in a restricted market (ham radio), G.E. published a paper showing 6L6GC making nearer 70 Watts at higher voltage and 5%-10% THD. Tube life may not be great in constant full-output operation, but hams speak in bursts and tolerate some distortion.

> that 450V on the plate, 400V on the screens, and a 6600 ohm plate to plate impedance will make 44 watts for a pair, thus 88 watts for a quad into 3300 ohms plate to plate impedance? The CHB100 OT is 2500 ohms impedance and B+ in mine was about 480V.

480V/450V, squared, is 1.138 times the power. Already at 100.12 Watts.

3300/2500 suggests 1.32 times the current (IF the tube will support it). Looks like 132 watts.

Take off 10% for OT loss, 118 Watts. Probably some B+ sag and maybe the tubes don't pass 1.32 times current, so slightly over 100W.

> Mine made 54 watts RMS into a resistive load

I'd re-test that. IF I really cared 50W or 100W. (Usually if 50W won't do the job, 100W isn't total bliss, so maybe not worth the effort.)

Bogen "had" to hit their specified numbers, because fixed-install contractors tended to own Test Gear (to impress customers, or just to play with) and maybe 10% of them actually load-tested the power amps they installed. In the fixed-install racket the equipment specs are considered almost as "sure" as a contract for 10,000 bricks. OK, you might overlook 9,999 bricks or 99W. But 5,400 bricks, or 54 Watts, would be cause for rejection.

PRR, I thought the 54 watts was a bit low, but I did a lot of playing around with it and that is all it would ever do. I was testing with an 8 ohm dummy load resistor connected to the 8 ohm tap, and taking the AC voltage at the output, squaring it, then dividing by the impedance. I looked at the output on the scope with a sine wave going into the input from my sig gen and backed off a tad from where I could see clipping. Is there a better way to test for output power than this?

The OT on the CHB100 is about the same size as one I have here that was meant for two 7591's, so I figured when the power was low, it must be because the small OT was limiting it. I am currently using the EH 7868's, but NOS ones didn't make a power difference and actually the EH ones seem to make a little more power and sound as good or better than the old ones. I am sure they won't last as long but they are certainly cheaper!

Until I see another CHB100 and can test it for power output stock, I guess I won't know if mine is a fluke or if there is something else wrong with it.

Greg